i8o PALEONTOLOGY 



There are certain important lessons which the study 

 of the enormous number of species of ammonites has 

 impressed upon palaeontologists. First is the danger of 

 basing any idea of relationship on external form alone. 

 Over and over again species of very similar form have 

 proved to differ widely either in their suture-details, or 



FlG. 52. MORTONICERAS INFLATOM (J. DE C. SOWERBY), 



ALBIAN (UPPER GAULT.) (x.) 



Ornament omitted in centre; bifurcating ribs replaced later by single 

 multituberculate ribs. Rostrum bent backwards. (After Buvignier. ) 



in their ontogeny (as shown by a study of the inner 

 whorls) : such species are said to be homceomorphs of one 

 another, and have more than once led, either to mistakes 

 in zonal stratigraphy, or to the erroneous idea that 

 certain ammonite-species have too long a range in time 

 to be of zonal value. 



Secondly, in spite of the richness of the ammonite- 



